1. Field of the Invention
The present invention relates to a motor control device for controlling operations of motors which are provided for driving a plurality of driven objects or units, respectively.
2. Description of the Background Art
A plurality of motors are often employed for driving a plurality of driven objects, i.e., objects to be driven in various fields. For example, an image reading apparatus arranged in a copying machine, image scanner or the like is provided with first and second carriages which are reciprocated in a secondary scanning (subscanning) direction for optically scanning an image on an original document laid on an original document table. The first carriage carries components such as a light source for illuminating the original image, and a reflector mirror for reflecting, toward the second carriage, the reflected light beams coming from the original image. The second carriage carries a reflector mirror for reflecting, in a predetermined direction, the image light beams coming from the first carriage.
In some of conventional apparatuses, a single motor is used for driving both the carriages. In this structure, a driving force is transmitted from the single motor to the first and second carriages via appropriate transmission devices such as belt or wire transmission devices for moving both the carriages at an intended speed ratio. In the driving operation for driving both the carriages by such transmission devices, an error is prone to occur at a light path length from the document surface to a focus position, for example, due to an error in assembly of the transmission devices, and/or extension of retained wires for transmission in the transmission devices. When the error once occurs, correction is difficult. Since the single motor drives both the carries, it operates under a heavy load, so that the carriages cannot be driven at a high speed (i.e., image cannot be scanned fast), and other problems may also occur. In view of this, such a structure have been proposed that employs a plurality of linear motors or rotary motors for independently driving the corresponding carriages, respectively.
In either of the foregoing structures, the second carriage is usually driven at half a speed of the first carriage during image scanning.
In the case where both the carriages are driven by the independent motors, respectively, however, variations in travel speed, travel distance and others occur due to independent driving itself as well as independent and synchronous driving of both the carriages under different loads. Therefore, it is difficult to ensure an intended speed ratio between the carriages as well as an intended positional relationship. This causes errors in focusing and others, so that it is difficult to perform precise image scanning and image reading.
Japanese Laid-Open Patent Publication No. 2-307385 (307385/1990) has taught a PLL speed control circuit for controlling and driving, at an intended speed ratio, two linear motors provided for two objects to be controlled, respectively. In this circuit, one reference pulse signal for controlling and driving the linear motors is divided (i.e., frequency-divided) to produce reference signals which are used for the two linear motors to drive them at the intended speed ratio and thereby control the speeds of both the motors based on these reference signals, respectively.
Japanese Laid-Open Patent Publication No. 58-7161 (7161/1983) has taught such a technique that two carriages in an image reading apparatus are driven by different pulse motors, respectively. These pulse motors are driven by the same control signal, and mechanical means is employed for attaining appropriate speed-reduction ratios and thereby attaining an intended speed ratio between the carriages.
Japanese Laid-Open Patent Publication No. 57-211171 (211171/1982) has taught such a technique that two carriages in an image reading apparatus are driven by different pulse motors in different driving manners, respectively. The different driving manners are achieved by employing different energizing manners for the pulse motors, respectively. A difference in step angle is utilized to attain the intended speed ratio between both the motors and therefore the intended speed ratio between both the carriages.
In the speed control circuit taught by the above Japanese Laid-Open Patent Publication No. 2-307385 (307385/1990), however, two controlled objects, i.e., objects to be controlled (e.g., two carriages in an image reading apparatus) are driven by different linear motors, respectively, and each linear motor is driven based on a reference signal produced independently of the other. Therefore, variation in speed occurs in each linear motor independently of the other, and thus variations in travel speed and travel distance occur in each of the controlled objects independently of the other. Therefore, it is difficult to ensure precisely an intended speed ratio and an intended positional relationship between the controlled objects.
In the carriage driving speed control taught by the foregoing Japanese Laid-Open Patent Publication No. 58-7161, the intended speed ratio between the carriages is ultimately attained by the reduction ratio attained by the mechanical means. Therefore, errors are prone to occur in the intended speed ratio and positional relationship between both the carriages due to an original error in the mechanical means itself and an error during use.
According to the manner taught by the foregoing Japanese Laid-Open Patent Publication No. 57-211171, in which two carriages are driven by different pulse motors and in different driving manners, respectively, the motors drive different carriages which applies different loads, respectively. This tends to cause a variation in speed of each motor, and therefore variations in the travel speed and distance of each carriage independently of the other. Therefore, it is difficult to ensure precisely the intended speed ratio and positional relationship between the carriages.